1. Field of the Invention
The invention pertains to a toric contact lens with the center of gravity displaced to its edge by making the lens thicker at one side, to a mould for producing the said lens and to a method of manufacturing the moulds.
2. Description of the Prior Art
Cutting or grinding off a lens section on the lower side of lens ("truncation") has been the method used almost exclusively to stabilize toric lenses against rotation in the eye and in a constant position of its cylindric (toric) axis according to the axis of astigmatism of the eye. The center of lens gravity is shifted upwards in this case and the lens would tend to turn by 180.degree. due to gravity unless the centre of lens gravity was shifted in the direction of the cut edge at the same time. This is realized, at present, almost exclusively by a prismatic shape superimposed to the whole lens, including its optical part, in such a way, that the lens is continuously thickened in all its parts towards the cut lower part (so called "prism ballast"). This leads to an excess of mass in the lower part and to a perfect stabilization of the lens in the eye, but a large thickness of the created edge causes, even if it is perfectly rounded, an unpleasant mechanical irritation of the lower eyelid which runs against this thick edge during blinking. In addition to this, the very optics of the lens is deformed by the addition of prismatic component to its refraction, thus shifting the perceived image in the vertical direction.
Another design of toric lens was proposed in the Czechoslovak Pat. No. 198,748 to obtain stabilization without above mentioned shortcomings. A soft lens was provided with thicker projections, which should shift the center of gravity to one side. However, this adaptation was successful only in part, because the proposed projections did not develop a sufficiently strong effect, which would provide a reliable stabilization of lens, and the projections of a larger size irritated the eyelid and caused undesirable motions by impact of the eyelid on the projections of recommended shape.